1. Field of the Invention
The present invention relates to an apparatus for and a method of driving an electroluminescence device by applying an alternating voltage to it.
2. Description of the Related Art
An electroluminescence device (Hereinafter, it is referred to as an "EL device".) is a type of emission device used for a back light of a liquid crystal panel.
The EL device has an emission layer and two electrodes, and the emission layer is sandwiched between the electrodes. The emission layer includes a dielectric substance, in which a fluorescence substance is uniformly dispersed. If an alternating voltage is applied to the EL device, the alternating voltage is applied to the fluorescence substance in the emission layer, and the EL device emits light.
Further, the EL device has a characteristic that brightness increases in proportion to an amplitude and a frequency of the applied alternating voltage. For this reason, it is more suitable for driving the EL device to use an alternating current generated by converting a direct current, as compared with an use of an commercial alternating current.
In order to generate the alternating current by converting the direct current, an inverter and a direct voltage source are needed. Further, either a constant-current supply system or a constant-voltage supply system is generally used for controlling the direct current to be supplied to the inverter from the direct voltage source. The constant-current supply system is a system in which a direct current having a constant current is always supplied to the inverter. On the other hand, the constant voltage supply system is a system in which a direct current having a constant voltage is always supplied to the inverter.
As shown in FIGS. 6A and 6B, a characteristic of the brightness of the EL device varies depending on whether either the constant-current supply system or the constant-voltage supply system is used for controlling the direct current to be supplied to the inverter.
As shown in FIG. 6A, in the case that the constant-voltage supply system is used, there is an advantage that the brightness is stable at the beginning of driving the EL device. However, there is a disadvantage that the brightness decreases with a driving time of the EL device, with the result that the lifetime of the EL device (i.e. the period of time in which the EL device keeps the effective brightness) is short. This is because the EL device has a characteristic that the impedance thereof increases with the driving time. When the impedance of the EL device increases, a driving current to be used for driving the EL device decreases in the case of the constant-voltage supply system. Therefore, the brightness of the EL device decreases, and the effective brightness cannot be kept for a long time.
On the other hand, as shown in FIG. 6B, in the case that the constant-current supply system is used, there is an advantage that the effective brightness can be kept for a long time. Namely, when the impedance of the EL device increases with the driving time of the EL device, a driving voltage to be used for driving the EL device increases in accordance with the increase in the impedance of the EL device. Therefore, the brightness of the EL device is compensated by the increase in the driving voltage. This is because the current supplied to the inverter is always constant in the case of the constant-current supply system.
However, as shown in FIG. 6B, in case that the constant-current supply system is used, there is a disadvantage that the brightness of the EL device sharply increase to a high brightness exceeding the normal brightness at the beginning of driving the EL device. Namely, the EL device has a characteristic that the impedance thereof sharply increases at the beginning of driving. Therefore, the driving voltage sharply increases with the increase in the impedance of the EL device at the beginning of driving. As a result, the brightness of the EL device sharply increases at the beginning of driving. Thereafter, the brightness slowly decreases to the normal brightness with the decrease in the impedance of the EL device.
Further, if the brightness of the EL device becomes high, a deterioration of the EL device relatively becomes fast. Accordingly, the sharp increase in the brightness of the EL device at the beginning of driving makes the deterioration of the EL device fast.